1. Field of the Invention
This invention relates to an apparatus for determining the density of a liquid or a gas. The apparatus has an oscillator body filled with a test sample of the liquid or gas being measured. A sensor outputs signals representative of oscillations of the oscillator body which are amplified by an oscillation amplifier. The amplifier signals are supplied to a actuator disposed on the output side of the oscillation amplifier to drive the oscillator.
2. Description of the Related Art
Such an apparatus was disclosed, for example, in Austrian Letters Patent 331,539. In this apparatus a circuit for excitation and detuning is provided. The circuit essentially consists of an amplifier and a limiter as well as a further circuit for the temperature-dependent de-tuning of the resonance frequency of the oscillator.
The latter consists of a temperature-dependent resistance bridge, a summing amplifier and integrators for phase shifting.
Thus, it is possible with this known device to obtain the excitation of the oscillating body and de-tuning of the resonance frequency, by means of which a temperature compensation and/or a counter mass compensation can be achieved, the latter in order to reduce a "counter mass--spring--measuring oscillator" model of a coupling oscillator system, which is the basis for calculating the density, to the theoretically ideal, simple "spring --oscillating mass" oscillator system.
The oscillator with the sample to be tested contained in it represents an oscillating body which is embodied in such a way that a defined volume of the test sample to be tested takes part in the oscillation and thus affects the period length .tau.. In this case the period length .tau. of such an oscillating body can be calculated in accordance with equation (1): ##EQU1## where M.sub.o is the mass of the empty oscillator, c its resilient constant, .rho. the density and V the volume of the test sample participating in the oscillation. The quotients M.sub.o /c and V/c can be considered to be oscillation-specific apparatus constants.
Following transformation and definition of simple constants A and T.sub.o, the following equation (2) is obtained: ##EQU2## where T is a whole number multiple of the period length .tau., T.sub.o is a multiple of the period length .tau..sub.o of the empty oscillator, increased by the same factor. The factors A and T.sub.o are calculated by setting the equation (2) for two known values of .rho. and T and solving in accordance with the factors A and T.sub.o.
If the oscillating body, as in the known solution, consists of an oscillating tube, clamped on one or both ends, in which the substance to be measured is enclosed, the equations (1) and (2) are only valid for non-viscous liquids. A larger period is observed with viscous liquids of a defined density.